Well I don't think you will be able to use an exhaust that will be good enough for both N/A and Turbo (if you decide to convert it in the future), but most people don't go larger than 2" diameter for N/As Have you got any silencers at the moment or is it straight through from the cat? If it's completely straight through then that's probably why it's really raspy I'm planning to get a Cat-back system, "Straight through" centre section to a 4" Back box. Another member on here, Sparky had this setup on his N/A and it sounded awesome! There's a clip of it at the bottom of this page Clicky!